Dollar Spot
By: Sandy Feather
©2008
Penn State
Cooperative Extension
Q.
I have some dead spots in my backyard that I attributed to the dry
weather. After two days of spotty rain, I saw this white mold over
all of the dead spots, but not on healthy grass. Is it something to
be concerned about, or will it go away when it cools off and we get
more rain?
A. Dollar Spot
is most severe on Kentucky bluegrass (Poa pratensis). The causal
fungus, Sclerotinia homeocarpa, is believed to overwinter in dormant
turf that was infected with dollar spot during the previous growing
season. Dollar spot development is favored by air temperatures in
the 80°F range and high humidity. It usually becomes active from
late spring through early summer, and can remain active all summer,
right into fall.
Dollar spot spreads on infected clippings, and can be carried by
lawn equipment, foot traffic, and can also be transported by the wind. The
disease starts out with tan colored patches of lawn that are two to
four inches in diameter. As it progresses, those spots coalesce
to create large patches of dead grass. White mycelium or fungal
strands are sometimes visible early in the morning when the grass is
wet with dew - it looks as though the lawn is covered with cobwebs.
Infected blades of grass may have tan spots that have an hourglass
shape with dark brown margins. Dollar spot is most severe on lawns
suffering from nitrogen deficiency and those that are watered
lightly every day or every other day. However, it can also develop
on non-irrigated lawns when we have long periods of warm, humid
weather.
Cultural controls include maintaining adequate fertility levels,
preferably by using fertilizers with a high percentage of slow
release nitrogen. Although fertilizing non-irrigated lawns in hot,
dry weather is a waste of money, those that are irrigated should
receive a moderate application of nitrogen in mid-summer. One-half
pound of slow release nitrogen around July 15 should carry the lawn
through until the late summer application in early to mid-September.
It is important to water deeply and infrequently, applying an inch
of water a week in one or two long-soaking sessions. Avoid watering
at night, which keeps the grass wet longer than watering first thing
in the morning. If dollar spot is a recurring problem on your lawn, overseed or replace susceptible Kentucky bluegrass with resistant
cultivars. They include A-34, Banff, Eagleton, NuStar, Preakness,
Princeton 104 and Unique.
Some fungicides that are readily available
to home gardeners and labeled to control dollar spot include
Cleary's 3336 (thiophanate methyl), Bayleton (triadimefon), and
those containing mancozeb as the active ingredient. Mancozeb is
preferred because it works on contact and fungi are less likely to
build up resistance to contact-type fungicides. Systemic fungicides
such as Cleary’s 3336 and Bayleton can lose effectiveness over time
as the target fungi develop resistance to them.
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